1. Field of the Invention
The present invention relates to a solar cell, in particular, a solar cell including a compound semiconductor layer comprising at least one element from each of groups Ib, IIIb and VIb as the light-absorption layer.
2. Description of the Prior Art
CuInSe.sub.2 and Cu(In, Ga)Se.sub.2 are compound semiconductors (having a chalcopyrite structure) comprising at least one element from each of groups Ib, IIIb and VIb. Thin-film solar cells using a CuInSe.sub.2 film (hereinafter, also referred to as a CIS film) or a Cu(In, Ga)Se.sub.2 film (hereinafter, also referred to as a CIGS film) as the light-absorption layer have a high energy conversion efficiency, which does not deteriorate due to irradiation or the like. Therefore, such a thin film solar cell has received attention.
In the solar cell, theoretically, when the band gap of the light-absorption layer is in the range from 1.4 eV to 1.5 eV, the highest conversion efficiency can be obtained. In the case of the solar cell including a CIGS film, it is possible to control the band gap by changing the ratio of Ga and In. When the ratio Ga/(In+Ga) by number of atoms is in the range from 0.5 to 0.8, the band gap is 1.4 eV to 1.5 eV.
However, in the current CIGS solar cells, the highest conversion efficiency can be obtained when the band gap of the CIGS film is in the range from 1.2 eV to 1.3 eV (corresponding to the range of the ratio Ga/(In+Ga) by number of atoms from 0.2 to 0.3). In the current CIGS solar cell, contrary to the theory, even if the band gap is broadened by increasing the Ga concentration, the conversion efficiency is reduced.
The CIGS solar cell with high conversion efficiency that has been reported has a heterojunction of a CdS film as the window layer and a CIGS film as the light-absorption layer. On the other hand, in recent years, a CIGS solar cell without CdS has received attention for environmental reasons. As a result, several CIGS solar cells using a ZnO based semiconductor as the window layer instead of CdS, have been reported. However, these cells have a lower conversion efficiency than that of cells comprising the CdS film. When a ZnO based semiconductor is used as the window layer, especially the open-circuit voltage is low.